The present invention is directed toward the field of cellular communications systems and methods of operating the same. In particular, a method for maximizing the spectral efficiency of the available carrier frequencies in a cellular system is disclosed. As used in this application, the term spectral efficiency refers to the traffic capacity of a particular system based on available carrier frequencies. Put another way, for a given number of carrier frequencies, a system that can carry more traffic has greater spectral efficiency. The method of the present invention maximizes system traffic capacity (and hence the spectral efficiency) by assigning multiple overlapping frequency reuse patterns to a cellular grid consisting of a plurality of adjoining outer cells, and at least one sub-group of inner cells or sub-cells contained within the outer cells. The inner cells of the present invention are interference-limited as opposed to path-loss or signal-strength limited and may be generated using the same equipment as the outer cells. This method can be applied to any cellular system that employs mobile units capable of directly measuring the interference level of the serving cell and adjacent or neighboring cells in order to provide a fast mobile-assisted handoff (MAHO) as the mobile unit moves from the outer cell grid to the inner cells and vice versa.
In any cellular system the spectral efficiency of available carrier frequencies is important since spectral efficiency correlates to traffic capacity. Generally, cellular network providers are assigned a limited number of carrier frequencies over which they can operate their systems. So, it becomes critical for the network provider to maximize the efficiency in which these assigned carrier frequencies are used. The more often a particular carrier frequency can be reused in the system, i.e. the more often the system can use the same carrier frequency for multiple communications with mobile units being serviced by the system, the more efficient the system is and the more traffic it can carry. Therefore, it is desirable in any cellular system to be able to maximize the reuse of the available carrier frequencies. Prior art systems have not provided an effective technique for maximizing carrier reuse.
Most cellular systems generate a grid of adjacent cells (outer cells), each outer cell being generated by a separate base station transceiver. A cell is the geographic area over which a particular base station can effectively communicate with mobile units in the system. The base station transceiver includes an antenna, transmitting and receiving circuitry, and circuitry for connecting the base station to at least one network controller that manages the overall operation of the cellular system. One example of such a cellular system is the iDEN network, built by Motorola, Inc. (See, Technical Overview Notes on the iDEN System, Motorola Document, PRO-PCP-026, Version R01.00.02, May, 1995, for an overview of this digital cellular system.)
iDEN, like many other cellular systems, provides base station transceivers for generating a grid of outer cells. Mobile units (subscribers) initiate calls within the system by communicating to a nearby base station that sets up the call. The base station that a mobile unit is presently communicating with is known as the serving base station. If the mobile unit is travelling about the cellular system, such as a subscriber in a car, it becomes necessary to detect when the communication signal from the mobile unit is degrading and switch the mobile unit to another outer cell. Mobile units in the iDEN system measure the received signal strength (RSSI), or power, from the serving cell and from adjacent neighboring outer cells, and also directly measure the carrier to interference ratio (C/I) of the serving cell and adjacent neighboring cells. The C/I measurement, or interference measure, is made in iDEN using Motorola's patented (U.S. Pat. No. 5,490,177) Symbol Quality Estimate (SQE), which is a fast, direct measure of interference in a QAM modulated digital system. The mobile units periodically take these measurements, and if handoff criteria programmed into the system are met, a report is transmitted to the network controller to trigger a handoff to another cell. Although the iDEN system provides mobile assisted direct measurement of signal strength and interference, the system does not provide an effective technique for maximizing frequency reuse of available carrier frequencies in order to maximize traffic capacity of the system.
In the iDEN system, as in other prior art cellular systems, a grid of outer cells is provided. Because of interference levels between adjacent outer cells, a given carrier frequency cannot be used in immediately adjacent cells. In fact, specific carrier frequencies must be spaced apart by many cells in order to be effectively reused in these systems with an acceptable interference level. For example, the iDEN system typically employs a frequency reuse pattern with a cluster size of 17 cells. The cluster size measurement is also known as the "K-value". With a cluster size of 17, a particular carrier frequency can be reused only every 17 cells. This is a substantial problem in urban areas where the number of mobile units is high, and the number of available carriers may not be sufficient to support the user traffic when each carrier can only be reused every 17 to 20 cells in the cellular grid and the cells are typically spaced miles apart. When channels are unavailable calls are blocked, i.e. not allowed into the system, or may be dropped, i.e. a connected call is disconnected. Either case is an undesirable scenario for the cellular system provider. So, a method of maximizing carrier frequency spectral efficiency of such cellular systems is needed.
Prior art attempts to maximize the spectral efficiency of available carrier frequencies generally include schemes that require additional hardware (or base stations) in order to operate the method. Two examples of such systems are the "underlay/overlay" method and the "microcell" method. In the underlay/overlay method, a path-loss limited inner cell is positioned within each outer cell of the cellular grid. This is accomplished by placing an additional base station transceiver at the exact location as the transceiver that is generating the outer cell. But, the second base station transceiver is placed at a lower altitude than the outer cell base station in order to increase path-loss, and hence increase C/I. The problem with this type of system is that because the inner-cell is limited by path-loss or RSSI, and because the mobile units in such systems directly measure only the RSSI parameter, it is possible that the system will switch the mobile unit to a cell where the RSSI might be acceptable but the interference level (C/I) from an adjacent cell using the same carrier frequency is not acceptable. In order to compensate for this possibility, radio planners designing such an underlay/overlay system must set the handoff path-loss threshold such that the mobile will not switch to a cell with unacceptable C/I. This requires the radio planner to introduce a high degree of fading margin into the design of the inner cell, thus yielding an inefficient use of available carrier frequencies, and hence a less-than-maximum traffic capacity for the cellular network.
Another type of prior art underlay/overlay type of system also employs path-loss limited inner cells, and then attempts to calculate or estimate the C/I based upon the mobile unit's measurement and reporting of RSSI. Although such a system can yield improved performance over the traditional underlay/overlay system, fading margins must still be implemented, and if the mobile unit is moving at a fast rate of speed, then the extra time required to perform the complex C/I calculation may be too great for the network controller to be able to make the switching decision in a timely fashion.
Still another type of prior art system for improving frequency reuse is a microcellular system in which inner cells are added to the outer cells, but not at the same location as the outer cell. These systems require the addition of hardware to the cellular network to support the inner microcells, and also suffer from the same problems as the underlay/overlay systems in terms of being limited to measuring RSSI in order to make a handoff decision, and the associated efficiency limitations of such RSSI-only systems.
A common shortcoming with the traditional underlay/overlay and microcellular approaches is the use of path-loss limited sub-cells. The switching boundary of these sub-cells is set by the RSSI level at which the system has been programmed to handoff. These types of systems do not permit the use of overlapping frequency reuse schemes having tight reuse patterns because they have no means for directly measuring the interference from neighboring sub-cells. The RSSI-only measurement will not assume that a cell has adequate noise immunity from nearby sub-cells, only that there is sufficient signal strength. Therefore these systems may perform a handoff to a cell that has an unacceptable level of interference, which translates into poor call quality.
Therefore, there remains a need in this art for a method and system for maximizing spectral efficiency of a cellular network. More particularly, there remains a need for such a system that can be implemented in existing outer cell-only cellular systems, such as iDEN, without having to add additional hardware to the system. However, such a method could be combined with traditional methods for increasing the carrier frequency reuse efficiency in a cellular system, such as underlay/overlay or microcellular approaches.
There remains a more particular need for a cellular system employing overlapping multiple frequency reuse patterns with outer cells and at least one level of inner cells, wherein carrier frequencies are reused more efficiently on the inner cells than in the outer cells.
There remains an additional need for such a system in which the inner cells are not limited or bounded by path-loss or RSSI, but rather are limited by the actual interference generated by adjacent inner cells, and in which the mobile units of the system are able to directly measure the interference of the serving cell and nearby neighboring cells in order to perform a mobile-assisted handoff (MAHO) with the network controller.